The present specification relates to displays. More particularly, the present specification relates to head up displays (HUDs).
HUDs can be used in a variety of applications. In aircraft applications, HUDs can provide significant safety and operational benefits including precise energy management and conformal flight paths. These safety and operational benefits are enjoyed by operators of air transport aircraft, military aircraft, regional aircraft and high end business jets where HUDs are generally employed. These safety and operational benefits are also desirable in smaller aircraft.
Conventional HUDs generally include a combiner assembly and optics for projecting information to a combiner disposed in the combiner assembly. A conventional stow mechanism can be attached to the combiner assembly and used to rotate the combiner about a single axis to and from a stowed position and an operational position. In the stowed position, the combiner is in a position that does not obstruct the pilot, especially during ingress and egress to and from the pilot's seat in the cockpit. In addition, the stow mechanism can include a break away mechanism which positions the combiner away from the pilot in the event of a catastrophic event.
Conventional HUDs require that the alignment between the combiner and the projection optics be monitored to prevent misalignment errors. Small deflections in the position of the combiner with respect to the projection optics can cause significant alignment errors associated with the information or symbology projected by the optics onto the combiner. Alignment errors associated with symbology and its placement in the real world view can result in misleading information. Imprecision in the stow mechanism can contribute to alignment errors when the combiner is moved to and from the operational position and the stowed position.
According to one conventional system, an optical monitor can be employed to detect alignment errors. One conventional technique employs a light emitting diode (LED), a mirror and a photosensitive diode to form a Combiner Alignment Detector (CAD). A conventional CAD is discussed in U.S. Pat. No. 4,775,218.
The LED and photosensitive diode of the CAD can be mounted on the fixed part of the combiner assembly and the mirror can be mounted on the moving portion of the combiner assembly. When the combiner is mis-positioned, a beam of light from the LED is deflected by the mirror and hits the photo diode off-center inducing an asymmetric signal that can be processed to calculate the error. If the error is too large, an ALIGN HUD message can be displayed.
CADs can be disadvantageous for a number of reasons. First, the CAD adds to the cost of the HUD and can be expensive to manufacture. Second, the CAD requires calibration which adds to manufacturing and service costs. Third, the CAD can be subject to failure. Fourth, a conventional CAD can give an erroneous ALIGN HUD message that may result from stray light, sunlight, dirt, or unknown electrical faults.
Therefore, there is a need for a HUD that does not require a CAD. Further, there is a need for a compact HUD which uses optics optimized for impunity to alignment errors. Yet further still, there is also a need for a small volume, lightweight, lower cost HUD. Yet further, there is a need for a substrate waveguide HUD with symmetrical couplers. Yet further, there is a need for a HUD with less angular and/or positional sensitivity. Yet further still, there is a need for a combiner configured so that a CAD is not required even when a less precise stow mechanism is utilized.